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Methods for Characterizing Near-Field Emissions from Energetics and Pyrotechnics
Dr. Kevin McNesby | U.S. Army Research Laboratory
WP-2611 intended to address an FY16 Strategic Environmental Research and Development Program (SERDP) SEED call for “Methods for Characterizing Near-Field Emissions From Metal-Based Energetics and Pyrotechnics WPSEED-16-01”. Researchers had originally intended to develop methods to predict and quantify emissions of gases and particles produced by functioning (i.e., exploding) metal-containing solid explosives, gun propellants, and pyrotechnics. Early on in the work, it became apparent that the inclusion of pyrotechnics would carry the researchers beyond the one-year performance schedule, so the work was focused on metal-containing explosives and gun propellants. The research team believes the approach described in what follows was directly applicable to pyrotechnics.
Materials studied included M855 ammunition (nitrocellulose ((C6H7(NO2)3O5)n) /nitroglycerin (C3H5N3O9) propellant, Cu-jacketed lead projectile) fired using an M4 weapon; medium-sized (660 grams) charges of neat TNT (trinitrotoluene, C7H5N3O6); and medium-sized (660 grams) charges of TNT:Mg:B [by weight 80% TNT : 4% magnesium powder, 325 mesh : 16% boron powder, 0.8 micrometer]. Gun firings and explosions were conducted at the US Army Research Laboratory (ARL) at Aberdeen Proving Ground, MD. Emissions of gases and particles were measured using a mobile lab operated by the US Environmental Protection Agency (EPA), with chemical analysis by the EPA and the University of Dayton Research Center (UDRI). Predictions of emissions were performed at the ARL, and were based on the observed stages of energy release for each event.
For the gun firings, metal species produced by the event were found in particles. Carbon species produced by the event were found in gases and particles. The metal detected in highest concentration following the M4 carbine firing was copper (in the range of 50 grams per kilogram of propellant) which originated from the bullet casing rather than the propellant. Lead was detected for all tests, at approximately 1/3 the level of Cu. The source of the lead was believed to be the No. 41 primer used to initiate the propellant. The measured “modified combustion efficiency” (MCE), an indicator of the extent of carbon oxidation, was between 0.44 and 0.5 for all ammunition tested, indicating that the total initial carbon was under-oxidized (a MCE of 1 is full oxidation). This resulted in higher than expected concentrations of emitted CO gas.
This was in contrast to simulations, which predict approximately 25% higher level of carbon oxidation. As with the gun firings, metals in the explosive formulations were found after testing in the emitted particles, and carbon was found in the emitted gases and particles. Metals detected in highest concentration following detonation/explosion of “neat” (i.e., pure) TNT were iron (Fe) and aluminum (Al), likely from fixtures in the test environment and the blast chamber walls. For detonation/explosion of the TNT:Mg:B formulation, B and Mg were the metals detected at highest concentration after explosion, with measured particle masses approaching 300 grams per kilogram of explosive formulation. The experimental MCE for all detonations/explosions of “neat” and metallized TNT was near 0.98, indicating most of the carbon measured following explosion was fully oxidized. Simulations of the MCE for each material were within 25% of experiment.
Overall, the research team believes the main area for improvement in the gun emissions work is in the area of simulation of soot and particle production during interior ballistics, and in simulation of particle combustion during exterior ballistics. Of particular concern is the high level of CO gas measured following firing of the M4 carbine. For simulation and measurement of exploding metal-containing energetics, the main area for improvement is in prediction of particle combustion during the second stage of energy release, and eventual incorporation of heterogeneous reaction chemistry into fluid dynamic modeling. Researchers believe a follow-on effort should incorporate refinements in these areas, as well as others improvements detailed in the conclusions and recommendations section of the full report.